Preliminarily cooled flue gases from which particulates and sulfur trioxide have been removed are contacted with an SO.sub.2 absorbing liquid and the SO.sub.2 laden liquid is then regenerated by multi-stage vacuum stripping and returned in a closed cycle to the absorber. The absorbing liquid contains an additive and is a dilute solution of, preferably, calcium ions in water. The additive is added in minor amount sufficient to provide a partial pressure of SO.sub.2 in the absorber equivalent to the partial pressure of SO.sub.2 in water in the temperature range of about 15.degree. C. to -20.degree. C. The preliminarily cooled gas is at a temperature in the order of 125.degree. F. The heat removed from the closed cycle by vacuum stripping is substantially equal to the heat given up by the flue gas in the absorber, and the temperature difference between the liquid on entering and leaving the desorber is in the order of 10.degree.-30.degree. F.